1. Field of the Invention
The present invention relates to an optical disk capable of high density recording.
2. Description of the Related Art
As well known, in recent years, as an optical disk capable of high density recording of information, a DVD having a single layer per side capacity of 4.7 GB has been put into practical use. There exist DVD types such as a DVD-ROM for the exclusive use of reproduction, a rewritable DVD-RAM, and the like. A DVD is constructed in such a way that an information recording layer is formed on a transparent substrate (hereafter, referred to as a light transmission layer) having a thickness of 0.6 mm, and laser light is allowed to pass through the light transmission layer to converge on the information recording surface to write or read information. The numerical aperture (NA) of an objective lens for converging a beam of this time is 0.6 as a reference. The refractive index n of the light transmission layer is specified to be the range, n=1.45 to 1.65, with respect to the wavelength of 650 nm, and a light transmission layer material suitable for this condition is selected. As such light transmission layer material, polycarbonate is generally employed, and the refractive index in this case is n=1.58.
Although the reference of the thickness of the light transmission layer of a DVD is 0.6 mm as described above, it is unavoidable that a thickness dispersion occurs from the viewpoint of manufacturing of disks. In an optical system to record and reproduce a DVD, in the case where the light transmission layer is designed in such a way that the standard value of the thickness thereof is 0.6 mm, if the thickness of a substrate is manufactured departing from 0.6 mm, aberration occurs. Since such aberration of an optical system increases a beam spot diameter and adversely affects reproduction of a signal, it is necessary to restrain the aberration to a predetermined value or less from the viewpoint of the system.
The aberration of the optical system due to a thickness error of the light transmission layer is determined by both a deviation from a standard value of the light transmission layer and a deviation from a standard value of the refractive index of the light transmission layer. Accordingly, in the case of a DVD, in order to restrain the aberration of the optical system caused by a thickness error of the light transmission layer to a constant value or less, the range of the light transmission layer thickness is specified as a two-dimensional range of the light transmission layer thickness and the refractive index thereof. This range is disclosed for example in Jpn. Pat. Appln. KOKAI Publication No. 8-273199. That is, with respect to the range of the refractive index, n=1.45 to 1.65, in the case where the error of the light transmission layer thickness with respect to the standard value is ±0.03 mm, when a horizontal axis represents the refractive index and a vertical axis represents the light transmission layer thickness, if the refractive index n becomes smaller than a lens load specification (standard value), a range which is shifted in a direction in which the light transmission layer thickness is increased is specified, and if it becomes larger, a range by which the light transmission layer thickness is not changed is specified.
However, the specifications of the above-described publicly known example are not appropriate in view of the following.
Presently, technology development to make a DVD further high density has proceeded in various companies. The spot size of focused light emitted on the information recording surface of an optical disk is in proportion to the wavelength and is in inverse proportion to the NA showing the iris angle of an objective lens for focusing light. Accordingly, in order to contract the spot size of focused light, aiming at improving the recording density, it is necessary to shorten the light source wavelength and increase the NA of the objective lens.
At this time since the refractive index of the light transmission layer is dependent upon the light source wavelength, it is necessary to newly specify the range of the light transmission layer thickness as a two-dimensional range with its refractive index. As an example of the light source wavelength, the NA, and the light transmission layer thickness of a next generation optical disk, it can be shown that the wavelength λ=405 nm, NA=0.65, and the light transmission layer thickness=0.6 mm.